An electromagnetic fuel injector (for example of the type described in patent application EP1635055A1, which is incorporated by reference) comprises a cylindrical tubular body displaying a central feeding channel, which performs the fuel conveying function and ends with an injection nozzle regulated by an injection valve controlled by an electromagnetic actuator. The injection valve is provided with a needle, which is rigidly connected to a mobile keeper of the electromagnetic actuator between a closing position and an opening position of the injection nozzle against the bias of a spring which tends to maintain the needle in closing position. The valve seat is defined by a sealing element, which is shaped as a disc, lowerly and fluid-tightly closes the central channel of the support body and is crossed by the injection nozzle.
The driving time-injected fuel quantity curve (i.e. the law which binds the driving time to the quantity of injected fuel) of an electromagnetic injector is on a whole rather linear, but displays an initial step (i.e. displays a step increase at shorter driving times and thus at smaller quantities of injected fuel). In order words, an electromagnetic injector displays inertias of mechanical origin and above all of magnetic origin which limit the displacement speed of the needle and therefore an electromagnetic injector is not capable of performing injections of very reduced amounts of fuel with the necessary precision.
Conventionally, the capacity of performing fuel injections of very reduced duration with the necessary precision is expressed by a parameter called “Linear Flow Range” which is defined as the ratio between maximum injection and minimum injection in linear ratio.
Due to the relatively high “Linear Flow Range”, an electromagnetic injector may be used in a direct injection internal combustion engine in which the injector is not driven to inject small amounts of fuel; instead, an electromagnetic injector cannot be used in a direct injection internal combustion engine, in which the injector is constantly driven to inject small amounts of fuel so as to perform a series of pilot injections before the main injection (e.g. as occurs in an Otto cycle internal combustion engine provided with turbo charger).
In order to obtain an injector with a high “Linear Flow Range”, it has been suggested to use a piezoelectric actuator instead of the traditional electromagnetic actuator. A piezoelectric injector is very fast and thus display a high “Linear Flow Range”; however, a piezoelectric injector is much more expensive than an equivalent electromagnetic injector due to the high cost of piezoelectric materials. By way of example, the cost of a piezoelectric injector may even be three times the cost of an equivalent electromagnetic injector.
In order to obtain an injector having a high “Linear Flow Range” it has also been suggested to make a multipolar electromagnetic actuator instead of a traditional monopolar electromagnetic actuator; however, a multipolar electromagnetic actuator displays considerably higher production costs with respect to a traditional injector with monopolar electromagnetic actuator.